1. Field of the Invention
The present invention relates, in general, to the treatment of flue gas and, in particular, to a new and useful system and method for recovering useful heat from flue gas while removing particulates, fly ash, sulfur oxides and/or other contaminants contained in the flue gas formed during the combustion of waste materials, coal and other fossil fuels, which are burned by electric power generating plants, waste-to-energy plants and other industrial processes.
2. Description of the Related Art
There are several known systems that are used for the integrated heat recovery and pollutant removal of particulates, sulfur oxides and/or contaminants from a hot combustion exhaust gas in order to comply with government emissions requirements.
FIG. 1 shows one known system which is a condensing heat exchanger, generally designated 2, which recovers both sensible and latent heat from a flue gas 3 in a single unit. The gas 3 passes down through a heat exchanger 4 while water 6 passes upward in a serpentine path through the heat exchanger tubes. Condensation occurs within the heat exchanger 4 as the gas temperature at the tube surface is brought below the dew point. The condensate falls as a constant rain over the tube array and is removed at the bottom of the unit 2. Gas cleaning can occur within the heat exchanger 4 as the particulates impact the tubes and gas condensation occurs.
The heat exchanger tubes and inside surfaces of the heat exchanger shell 4 are made of, or covered with, a corrosion resistant material like TEFLON a registered trademark of Du Pont Co., in order to protect them from corrosion when the flue gas temperature is brought below the acid dew point. Interconnections between the heat exchanger tubes are made outside of the tube sheet and are not exposed to the corrosive flue gas stream 3.
FIG. 2 shows a second system which is an integrated flue gas treatment (IFGT) condensing heat exchanger, generally designated 10, designed to enhance the removal of pollutants from the flue gas stream 3. It is also made of corrosion resistant material or has all of the inside surfaces covered with TEFLON.
There are four major sections of the IFGT 10; a first heat exchanger stage 12, an interstage transition region 14, a second heat exchanger stage 16, and a mist eliminator 18. The major differences between the integrated flue gas treatment design 10 and the conventional condensing heat exchanger design 2 (FIG. 1) are:
1. the integrated flue gas treatment design 10 uses two heat exchanger stages 12 and 16 instead of one; PA1 4. the second heat exchanger stage 16 is equipped PA1 5. the mist eliminator 18 is used to separate the water formed by condensation and entrained from the sprays from the flue gas 3.
2. the interstage transition region 14, located between the two heat exchanger stages 12 and 16, is used to direct the gas 3 to the second heat exchanger stage 16, acts as a collection tank, and allows for treatment of the gas 3 between the stages 12 and 16;
3. the gas flow 3 in the second heat exchanger stage 16 is upward, rather than downward;
with an alkali reagent spray system 20; and
Most of the sensible heat and some latent heat is removed from the gas 3 in the first heat exchanger stage 12 of the IFGT 10. The transition region 14 can be equipped with a water or alkali spray system 20. The system 20 saturates the flue gas 3 with moisture before it enters the second heat exchanger stage 16 and also assists in removing sulfur pollutants from the gas 3. The transition section 14 is made of or coated with corrosion resistant material like fiberglass-reinforced plastic. The second heat exchanger stage 16 is operated in the condensing mode, removing latent heat from the gas 3 along with the pollutants. The top of the second heat exchanger stage 16 is equipped with the alkali solution spray system 20. The gas 3 in this stage 16 is flowing upward while the droplets in the gas 3 fall downward. This counter-current gas/droplet flow provides a scrubbing mechanism that enhances particulate and pollutant capture. The condensed gases, particulates, and reacted alkali solution are collected at the bottom of the transition section 14. The flue gas outlet of the IFGT 10 is equipped with the mist eliminator 18 in order to reduce the chance of moisture carryover.